In mice and humans, a minor population of B cells, B1 cells, is notable for its distinct array of cell surface antigens, its early appearance, its tissue distribution and its repertoire of antibody specificities. The antibodies produced are frequently auto reactive but there is no evidence that they are involved in autoimmune disease. B1 cells give rise to virtually all examples of human B cell CLL, B cell lymphomas in MDS patients and many murine B cell lymphomas. Two distinct hypotheses exist to explain the acquisition of different phenotypes. One states that the phenotypes are inherent in separate lineages of B cells (B1 and B2); they are determined in distinct precursors prior to the rearrangement of Ig genes. The phenotypes are fixed; B2 cells cannot be changed into B1 cells, nor vice versa. The other hypothesis states that commitment to a phenotype occurs after encounter with antigen; newly formed B cells, termed B0 cells, can be induced to differentiate along either of two pathways, termed B1 and B2, depending on the nature of the antigen. Multivalent antigen, leading to cross linking of surface Ig without involvement of T helper cells, induces the B1 phenotype, whereas cognate interaction with antigen and T helper cell induces the B2 phenotype. The majority of B cells are viewed as being virgin B0 cells that retain the capacity to differentiate along either pathway. This project is designed to resolve the issue by determining whether the B1 phenotype is programmed before or after expression of surface Ig. The immune response of mice to pyrrolidone (Py) will be characterized following immunization with the thymus independent (Ti) polymer, polyvinyl pyrrolidone (PVP36O) or the same hapten coupled to protein as a thymus dependent (TD) antigen. The phenotype of responding cells will be determined by FACS and ELISA spot analysis, and their derivation from fetal derived B cells or adult bone marrow by analysis of allotype chimeras. Antibodies produced in response to Ti and TD forms of Py will be characterized with respect to V gene usage, CDR3 assembly, isotype and the presence of somatic mutations, by analysis of hybridomas. A heavy chain VDJ assembly from a 1 light chain expressing hybridoma will be introduced into k chain deficient mice to yield a transgenic strain that produces large numbers of B cells bearing surface Ig reactive with Py. Immunizations will be done in vivo in transgenic mice and after B cell transfer to compatible recipients, as well as in vitro. Immunization with TD antigen will be supplemented with carrier specific T cells. Results from the project should demonstrate whether the strong neoplastic propensity of B1 cells is an inherent feature of a discrete lineage of cells or results from chronic antigenic stimulation of B0 cells with Ti antigens.